1. Pertinent Prior Art of Interest
To obtain sufficient accuracy, prior fiber optic gyros required a phase modulator and demodulator. For example, U.S. Pat. No. 4,410,275 which issued to H. J. Shaw, et. al on Oct. 18, 1983 for a FIBER OPTIC ROTATION SENSOR teaches the use of a modulator to phase modulate the counterpropagating waves in the fiber sensing coil about the Sagnac phase shift to increase the sensitivity of the instrument and to determine the direction of rotation of the instrument. The modulation occurs by modulating the length or refractive index of the fiber. A relative phase difference is produced in the counterpropagating waves by the difference in propagation time through the fiber loop as the loop turns. An output detector demodulates the signal to provide a measure of the angular rate of the sensing loop.
U.S. Pat. No. 4,468,085 issued Aug. 28, 1984 for a "Hybrid Optical Junction and Its Use in a Loop Interferometer" to Michel Papuchon, et al. See particularly FIGS. 2 and 6. The apparatus of the patent uses single mode fibers. FIG. 6 shows an embodiment wherein light from a light source 10 is delivered into a single mode fiber 2 which is one of a pair of substantially identical single-mode fibers. The light travels along the fiber 2, thence into a common single mode fiber. Light from the common single mode fiber is delivered, usually equally, into two substantially identical fibers, 6 and 7, then through fibers 26 and 30 into the angular sensing coil. Light emerges from the sensing coil and is delivered by leaders 30 and 26 back into fibers 6 and 7, thence into the common fiber. The common fiber delivers light into the fibers 2 and 3. It is detected by the detector 12 at 3.
2. Brief description of the Invention
The fiber optic gyro of this invention eliminates the modulator and demodulator found in state-of-the-art fiber gyros, and it is therefore inherently less expensive and less complex to manufacture.
Although the apparatus of the invention is described using optical waveguides mounted upon a substrate, it may use equivalent light carriers such as optical fibers.
First and second single-mode optical waveguides, substantially identical in cross section and length, are symmetrically coupled to a first end of a double-mode optical waveguide. The angles formed between the center lines of the first or second single mode guides and the center line of the duo-mode guide are large enough to separate the single mode guides, but should be as small as practical.
Third and fourth dis-similar single-mode optical wave guides are coupled to the second end of the double-mode optical waveguide to form mode splitters for splitting the two modes received from the duo-mode waveguide. The mode splitters are attached, as explained in an article entitled, "An analytic Solution for Mode Coupling in Optical Waveguide Branches" by William K. Burns, et. al., which appeared in the IEEE Journal of Quantum Electronics, Vol QE-16, No. 4, April 1980. The third and fourth waveguides are of different dimensions and depart at different angles from the double-mode waveguide.
A light source, preferably of coherent light or partially coherent light, is properly polarized to deliver, and delivers, light to the first waveguide. The first waveguide delivers single-mode light to the double-mode waveguide to excite both the fundamental and the second modes of the waveguide. The double-mode waveguide delivers the fundamental mode of energy to excite the fundamental mode of the fourth waveguide and delivers its second mode of energy to excite the fundamental mode of the third waveguide. The third and fourth waveguides deliver light to and receive light from opposite ends of a single mode optical-fibre rate-sensing coil.
The two signals travel from the third and fourth waveguides, through the sensing coil from one end to the other and depart, respectively, through the fourth and third waveguides. The returning signal in the third waveguide excites the returning fundamental mode in the duo-mode waveguide, and the returning signal in the fourth waveguide excites the returning second mode in the duo-mode waveguide.
The two modes in the double-mode waveguide travel at different speeds. If the two modes were to travel only the length of the guide, then reverse their direction, they would arrive back at the beginning of the guide in phase.
With the sensing coil, the relative phase of the two signals at the beginning of the duo-mode guide depends upon the sensed angular rate. The returning signal arriving at the first end of the double-mode waveguide, delivers part of the returned energy of each mode to the second waveguide and thence to an optical sensor.
Along the double-mode waveguide are positioned waveguide taps to extract energy from the double-mode waveguide. Preferably these taps are each positioned to extract, usually through single-mode waveguides, substantially equal amplitudes of light from the two modes within the waveguide. The extracted light is guided to additional detectors.
In one preferred embodiment, the distance between the entrances to two of the waveguides is substantially plus or minus multiples of pi, plus one-quarter of the beatlength (Dr. Kim define beatlength)between the two modes in the waveguide. With that spacing, the electrical signals from the two detectors are proportional to the sine and cosine of the angle sensed by the sensing coil. If the electro-magnetic fields within the waveguides were not disturbed by the additional taps, by the angles of injection and extraction of the light, and other disturbances within the waveguides, the optimum spacing between such entrances to the waveguides could be exactly multiples of pi, plus a quarter wavelength. However, with such disturbances, the exact position of the detectors for optimum performance needs to be determined by experiment.
It is therefore a feature and object of this invention to sense rotation and to produce signals that are measures of such sensed rotation.
Other features and objects will become apparent from the following description taken together with the accompanying drawing.